1. Field of the Invention
The present invention relates to a light source device, an optical scanning device, and an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus employing the Carlson process, for example, the surface of a rotating photosensitive drum is scanned by a light beam thereby forming an electrostatic latent image on the surface of the photosensitive drum. The electrostatic latent image is then developed into a visible image, i.e., a toner image, and the toner image is transferred onto a recording medium and fixed thereon. This type of image forming apparatus is often employed as an on-demand printing system for simple printing, and there is an increasing demand for a system capable of outputting a high-density image at high speed.
To satisfy such a demand, for example, Japanese Patent Application Laid-open No. 2006-88569 discloses an image forming apparatus including a light source, such as a surface emitting laser array (vertical cavity surface emitting laser (VCSEL)) that includes a plurality of light emitters, emits a plurality of light beams that simultaneously scan a plurality of scan lines on a scan surface. However, because the light emitters are two-dimensionally arranged in a monolithic manner, for example, density variation may be disadvantageously caused due to variation in the distance between adjacent beam spots formed on the scan surface and variation in the shape of the beam spots.
One approach to reduce such density variation is to adjust a relative position of an optical system and the light source. However, change in the relative position of the optical system and the light source changes intensity (light amount) of the light beam emitted from the light source and a relative position of the light source and a light receiving unit that monitors the light amount. If an intensity of the light beam emitted from the light source, or a relative position of the light source and a light receiving unit changes, the light receiving unit cannot detect the light beam accurately, which results in unequal outputs of the light emitters. This leads to quality deterioration of an image output such as density variation. Therefore, it is also required to perform optical initial adjustment such as optical axis adjustment between the light receiving unit and the light source. Such inconvenience also occurs when the light source is replaced.
Because the VCSEL (surface emitting laser) emits the light beams in a single direction perpendicular to an active layer, the light amount cannot be controlled by simple monitoring of the amount of light emitted backward, which is performed in an edge emitting laser. For example, Japanese Patent Application Laid-open No. H8-330661 discloses a light detecting device including a beam splitter fixed to a surface emitting laser. The beam splitter splits light beams emitted by the surface emitting laser such that a part of the light beams is deflected to and detected by a light detecting unit. However, because the beam splitter splits a flux of light beams, efficiency of using the light decreases. In addition, use of a beam splitter increases the cost. Japanese Patent Application Laid-open No. 2006-179769 discloses a semiconductor laser including a window that splits light beams emitted by a laser chip and a photodiode that is arranged outside an area of light beams passing through the window and that monitors a light amount. However, because elements and a pattern of the photodiode need to be arranged in a narrow area, it is difficult to manufacture such a semiconductor laser.
Because light beams emitted from light emitters formed on a VCSEL are modulated independently based on image data, it is required to individually control outputs (light amount) of the light emitters based on a result of detecting the light beams by a light receiving unit.